1. Field of the Invention
The invention relates to an electric power steering apparatus.
2. Description of the Related Art
An electric power steering apparatus (EPS) is equipped with a torque sensor that detects a steering torque transmitted via a steering shaft. The EPS applies assisting force to a steering system on the basis of the steering torque detected by the torque sensor.
Some of the above torque sensors for EPS are formed of a sensor element, such as a Hall integrated circuit (IC), of which the output level (output voltage) varies on the basis of the torsional angle of a torsion bar provided for the steering shaft. Then, for example, the torque sensor described in Japanese Patent Application Publication No. 2003-149062 (JP-A-2003-149062) includes dual Hall IC to improve reliability.
Independent two sensor signals are used to make it possible to easily and accurately determine whether the torque sensor is in failure. Specifically, for example, as shown in FIG. 5A and FIG. 5B, the torque sensor is set so that the output characteristics of the two sensor elements are inverse from each other, that is, signal waveforms (variation characteristics of output voltages Va and Vb) output for variations in steering torque are inverse from each other. Then, the total value Vab of the output voltages Va and Vb is monitored to determine whether the sensor elements of the torque sensor are in failure.
That is, unless at least one of the two sensor elements is in failure of some kind, as shown in FIG. 5C, the total value Vab of the output voltages Va and Vb of these sensor elements should be substantially constant at a predetermined voltage V2 that corresponds to approximately twice a predetermined voltage V1. The predetermined voltage V1 is a reference value of each of the output voltages Va and Vb (intermediate value of the values that the output voltages Va and Vb can range). Thus, when the total value Vab of the output voltages Va and Vb having such relationship exceeds an appropriate range (V2±α, “α” is an error margin), it may be determined that at least any one of the sensor elements is in failure.
Then, when a failure of at least any one of the sensor elements is detected in this way, application of the assisting force is immediately stopped for fail-safe. When the failed sensor element is identified, a temporary steering torque is calculated on the basis of the output signal of the other normal sensor element to continue power assist control.
However, the failed state may be identified from only the output voltages of the individual sensor elements only when the output voltage falls within an appropriate range in which the output voltage is determined to be normal (the range indicated by the region R1) or when the output voltage falls within a range in which the output voltage is definitely determined to be abnormal (when the output voltage falls outside a predetermined output range from a lower limit V3 to an upper limit V4, that is, the range indicated by regions R2 and R3), as shown in FIG. 6. Therefore, the condition that the failed sensor element may be identified is limited (in FIG. 6, the diagonally shaded areas are unidentifiable regions). When a failure of some kind occurs in any of the sensor elements that constitute the torque sensor, mostly, assisting force is steeply and gradually decreased. This steep gradual decrease in assisting force causes a steep change in steering feeling, which may probably make a driver discomfort.
A conceivable method for solving the above problem may be, for example, a method of monitoring a variation per unit time in the output voltage of each sensor element, as described in Japanese Patent Application Publication No. 2000-185657 (JP-A-2000-185657). A variation in steering torque caused by normal steering operation intrinsically has a limit. When the output voltage of any of the sensor elements steeply varies beyond a range corresponding to the limit, it is determined that a failure has occurred in that sensor element. Then, the signal output from the other normal sensor element is used to calculate the temporary torque, thus making it possible to increase the ability to continue applying assisting force after the failure in the torque sensor is detected.
However, the condition that the output voltage of any of the sensor elements steeply varies in this way occurs only when the failure is relatively severe as in the case where the output voltage falls outside the predetermined output range. Therefore, it is difficult to identify a failed sensor element of which a small error occurs to slightly exceed the appropriate range (see FIG. 6, the range indicated by the region R1) in which the output voltage is determined to be normal.